When the time comes for Baby Cambridge to marry, he or she will have to ask the monarch's permission, as the new law requires the first six people in the line of succession to obtain the monarch's permission to marry. Previously, under a law dating back to 1772, all descendants of George II had to seek the monarch's permission.

The new legislation was agreed in principle when it was proposed at the Commonwealth Heads of Government Meeting in Perth, Australia, in October 2011, shortly before the Duke and Duchess of Cambridge announced their engagement.

A more detailed agreement was reached at the following year's Commonwealth meeting in December 2012, days after the Duchess's pregnancy was announced.

The new law is still being ratified in the 15 other countries that have the Queen as their head of state, as they, too, must pass laws in their own parliaments which confirm Baby Cambridge will be accepted as their future monarch.

Ending the tradition of male primogeniture, or male precedence, required a complex set of amendments to some of the country's most important constitutional documents, including the Bill of Rights and Coronation Oath Act of 1688, the 1701 Act of Settlement and the 1707 Act of Union between Scotland and England.

Nick Clegg, the Deputy Prime Minister, who thrashed out the details of the new law, said at the time that it would "bring to an end centuries of discrimination against women".

But critics said the new law had not gone far enough, as they wanted to end the ban on Catholics ascending to the throne. One Scottish cardinal described the bar as "state sponsored sectarianism".

The law change also affects the line of succession among existing members of the Royal family, as the Queen's cousin, Prince Michael of Kent, who lost his place because of his marriage to a Catholic, is restored to the line of succession.

Before the Duke of Cambridge was born in 1982 a Labour MP, Michael English, tried to introduce a similar bill to Parliament but it failed to get a second reading following objections from other MPs.